1. Field of the Invention
The present invention relates to a PWM driver that switches an electric device on or off according to a PWM (pulse width modulation) signal.
2. Description of the Related Art
A PWM driver for controlling an electric device such as a motor or an electric heater sometimes encounters radio noises caused by high-frequency noises generated during PWM operation of the electric device. For example, a passenger of a vehicle may have a noise problem when the passenger is hearing music or some other program on a car audio set.
In order to solve this problem, JP-A-2005-185053 proposes to form the pulse shape of the PWM signal into a trapezoid whose rising and falling portions are controlled by the duty ratio thereof JP-A-2006-187173 proposes to change the cycle of a carrier signal according to a preset pattern or randomly. The above proposals aim to prevent the frequency spectrum of high-frequency noises from concentrating at the same frequency band.
However, more high-frequency noises are generated where the duty ratio of the PWM signal is lower than a lower threshold value or higher than a higher than a higher threshold value, as shown in FIG. 5.
It has been found that this is caused because the carrier signal is shaped into an impulse where the on-duty of the PWM signal is lower than a lower threshold value as shown in FIG. 4B, or the on-duty of the PWM signal is higher than a higher threshold value as shown in FIG. 4C.
In this case, the above-described PWM drivers can not reduce these high frequency noises.